1. Technical Field of the Invention
The present invention relates to earth boring bits, and more particularly to those having rotatable cutters, also known as rotary cone drill bits.
2. Description of Related Art
Reference is made to FIGS. 1 and 2, wherein FIG. 1 illustrates an isometric view of a prior art rotary cone drill bit 10 and FIG. 2 illustrates a cross-sectional view of a portion of the prior art rotary cone drill bit 10 of FIG. 1. A leg 12 depends from a body portion 14 of the drill bit 10. The leg 12 includes a bearing shaft 16 which extends in a downward and radial inward direction. The bearing shaft 16 includes a cylindrical bearing surface 18. A cutter cone 20 is mounted to the bearing shaft 16 and supported for rotation by the bearing surface 18. In an alternative implementation, the cutter cone 20 is supported for rotation on the bearing shaft 16 by a set of roller bearings. The shape and configuration of the cone 20, as well its rotatable attachment to the bearing shaft 16, is well known in the art. In sealed bearing implementations, the bearing (journal or roller) between the cone 20 and bearing shaft 16 is lubricated by a lubricant (such as a grease) that fills regions adjacent to the bearing as well as other passages 21 in the rotary cone drill bit in a manner well known by those skilled in the art. This lubricant is retained within the rotary cone drill bit through the use of, for example, a resilient seal in the form of an o-ring 22 positioned in a seal gland 24 between the inner cylindrical surface 26 near the base of the cone 20 and the outer cylindrical surface 28 near the base of the bearing shaft 16.
The outer surface 30 of the leg 12 terminates at a semicircular edge 32 proximal to the cone 20. The region of the leg 12 associated with the surface 30 is known in the art as the “shirttail region,” and the edge 32 is known in the art as the “shirttail edge.” The shirttail edge 32 is provided where the terminal portion of the surface 30 transitions to an inside radial surface 34 oriented parallel to the base of the cone 20 (and perpendicular to the bearing shaft 16) and positioned at the base of the bearing shaft 16. On a rotary cone drill bit 10, one of the primary forms of bit failure can be traced back to shirttail wear. In one form of such shirttail wear, the shirttail edge 32 wears down, the radial extent of the inside radial surface 34 is decreased by this wear, and the resilient o-ring 22 seal in sealed bearings is exposed. If the bearing is instead an open (non-sealed or air) bearing, the wearing of the shirttail edge may expose the air bearing. Another form of shirttail wear includes wear of the shirttail outer surface 30 at locations away from the shirttail edge.
The prior art teaches two methods for delaying shirttail wear. FIG. 3 illustrates a first method in which a layer of welded hardfacing material 40 is applied to the surface 30 extending along at least a portion of the shirttail edge 32. The hardfacing material is typically a deposit of tungsten carbide hardmetal 40 applied to the surface 30. The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material 40 wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material. FIG. 4 illustrates a second method in which tungsten carbide inserts 42 are press-fit into holes 44 formed in the surface 30 near the shirttail edge 32. While these inserts 42 provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts 42 do not provide protection for the shirttail edge 32. The reason for this is that the holes 44 must be located at some appreciable distance from the shirttail edge 32 in order for the press-fit to function properly and peripherally retain the inserts 42. For example, a separation d1 of at least 0.125 inches is typically provided from the edge of the hole 44 to the shirttail edge 32. Thus, the method of FIG. 4 functions to primarily protect the shirttail region near to, but not exactly at, the shirttail edge 32. Furthermore, in order to be suitably retained, the press-fit inserts 42 must typically have a thickness t (with a corresponding depth of the hole 44) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5)
A need accordingly exists in the art to provide an improved method of protecting the shirttail edge 32.
With reference once again to FIGS. 1 and 2, the outer surface 30 of the leg 12 in the shirttail region laterally terminates at a leading shirttail edge 50 and a trailing shirttail edge 52. The leading shirttail edge 50 is especially susceptible to wear during operation of the rotary cone drill bit 10. The prior art again teaches two methods for delaying wear of the leading shirttail edge 50. FIG. 5 illustrates a first method in which a layer of welded hardfacing material 40 is applied to the surface 30 extending along at least a portion of the leading shirttail edge 50. The hardfacing material is typically a deposit of tungsten carbide hardmetal 40. The material is typically pelletized tungsten carbide carried in a nickel welding medium. This solution does not work well when the rotary cone drill bit is run in a highly abrasive rock formation because the hardfacing material 40 wears down too quickly. It is primarily the welding medium, typically nickel, which accounts for the relative poor performance of the weld on material. FIG. 6 illustrates a second method in which tungsten carbide inserts 42 are press-fit into holes 44 formed in the surface 30 near the leading shirttail edge 50. While these inserts 42 provide better abrasion resistance (in comparison to the use of hardfacing material), the inserts 42 do not provide protection for the leading shirttail edge 50. The reason for this is that the holes 44 must be located at some appreciable distance from the leading shirttail edge 50 in order for the press-fit to function properly and peripherally retain the inserts 42. For example, a separation d2 of at least 0.125 inches is typically provided from the edge of the hole 44 to the leading edge 50. Thus, the method of FIG. 6 functions to primarily protect the shirttail region near to, but not exactly at, the leading shirttail edge 50. Furthermore, in order to be suitably retained, the press-fit inserts 42 must typically have a thickness t (with a corresponding depth of the hole 44) such that a ratio of the thickness of the insert to a diameter d′ of the insert (where the inserts are round) or width w of the insert (with other shapes) exceeds about 0.5 (i.e., t/d′≧0.5; or t/w≧0.5)
A need thus exists in the art to provide an improved method of protecting the leading shirttail edge 50.